1. Field of the Invention
This invention relates to an improved process combination for the conversion of hydrocarbons, and more specifically for the selective upgrading of naphtha fractions by a combination of aromatics removal and selective isoparaffin synthesis.
2. General Background
The widespread removal of lead antiknock additive from gasoline and the rising fuel-quality demands of high-performance internal-combustion engines have compelled petroleum refiners to install new and modified processes for increased "octane," or knock resistance, in the gasoline pool. Refiners have relied on a variety of options to upgrade the gasoline pool, including higher-severity catalytic reforming, higher FCC (fluid catalytic cracking) gasoline octane, isomerization of light naphtha and the use of oxygenated compounds. Such key options as increased reforming severity and higher FCC gasoline octane result in a higher aromatics content of the gasoline pool, through the production of high-octane aromatics at the expense of low-octane heavy paraffins. Current gasolines generally have aromatics contents of about 30% or higher, and may contain more than 40% aromatics.
Currently, refiners are faced with the prospect of supplying reformulated gasoline to meet tightened automotive emission standards. Reformulated gasoline would differ from the existing product in having a lower vapor pressure, lower final boiling point, increased content of oxygenates, and lower content of olefins, benzene and aromatics. The oxygen content of gasoline will be 2 mass % or more in many areas. Gasoline aromatics content is likely to be lowered into the 20-25% range in major urban areas, and low-emission gasoline containing less than 15 volume % aromatics is being advocated for some areas with severe pollution problems. Distillation end points also could be lowered, further restricting aromatics content since the high-boiling portion of the gasoline which thereby would be eliminated usually is an aromatics concentrate. End point often is characterized as the 90% distillation temperature, currently limited to a maximum of 190.degree. C. and averaging 165.degree.-170.degree. C., which could be reduced to around 150.degree. C. in some cases.
Since aromatics have been the principal source of increased gasoline octanes during the recent lead-reduction program, severe restriction of the aromatics content and high-boiling portion will present refiners with processing problems. Currently applicable technology includes such processes as recycle isomerization of light naphtha and generation of additional light olefins through fluid catalytic cracking and isobutane through isomerization as feedstock to an alkylation unit. Increased blending of oxygenates such as methyl tertiary-butyl ether (MTBE) and ethanol will be an essential part of the reformulated-gasoline program, but feedstock supplies will become stretched. Selective isoparaffin synthesis to produce desirable gasoline components is known but has not yet become attractive for commercialization.
A process designated as "I-cracking" for increasing the yield of naphtha and isobutane is disclosed in U.S. Pat. No. 3,692,666 (Pollitzer). U.S. Pat. No. 3,788,975 (Donaldson) teaches a combination process for the production and utilization of aromatics and isobutane. The combination includes selective production of isobutane from naphtha followed by a combination of processes including catalytic reforming, aromatic separation, alkylation, isomerization, and dehydrogenation to yield alkylation feedstock. The paraffinic stream from aromatic extraction is returned to the I-cracking step. Neither Pollitzer nor Donaldson disclose the present process combination, however, nor do they recognize any problem from processing aromatics-containing charge stocks.
A combination process for gasoline production is disclosed in U.S. Pat. No. 3,933,619 (Kozlowski). High-octane, low-lead or unleaded gasoline is produced by hydrocracking a hydrocarbon feedstock to obtain butane, pentane-hexane, and C.sub.7 + hydrocarbons, and the C.sub.7 + fraction may be sent to a reformer. U.S. Pat. No. 4,647,368 (McGuiness et al.) discloses a method for upgrading naphtha by hydrotreating, hydrocracking over zeolite beta, recovering isobutane, C.sub.5 -C.sub.7 isoparaffins and a higher boiling stream, and reforming the latter stream. These references do not teach or suggest the present process combination, however.
Isomerization of C.sub.4 -C.sub.6 paraffins with a hydrogenation zone upstream to saturate benzene is taught in U.S. Pat. Nos. 3,759,819 (Attane et al.) and 5,003,118 (Low et al.). However, neither Attane et al. nor Low et al. teach selective isoparaffin synthesis yielding superequilibrium isoparaffins in excess of the concentration that would be achieved by isomerization or suggest that aromatics saturation may be applied in that context.
The prior art, therefore, contains elements of the present invention. There is no suggestion to combine the elements, however, nor of the surprising benefits in selectivity that accrue from the present process combination to obtain intermediate hydrocarbons suitable for producing reformulated gasoline.